The long-term objectives of Project 2 are to identify novel neurobiological substrates of addiction that will lead to therapeutic advancements in concert with the other NARC projects. Three major findings have emerged from Project 2 in the current budget period. First, an infusion of brain-derived neurotrophic factor (BDNF) into the dorsomedial prefrontal cortex (dmPFC) immediately after the last cocaine self-administration session suppresses extinction responding, cue-, and cocaine-induced reinstatement of cocaine seeking. Although the precise mechanism is unknown, preliminary data suggest that exogenous BDNF is anterogradely transported to the nucleus accumbens (NAc) where it normalizes cocaine-induced alterations in extracellular glutamate and phospho-ERK expression. Second, the expression of a subset of mRNAs that are enriched in dendritic polysomes is altered in the mPFC 22 hr or 3 weeks after the end of cocaine selfadministration. Some of these mRNAs, including BDNF, the protein phosphatase 1 binding protein, spinophilin, protein phosphatase 1 catalytic subunit beta, and the double-stranded RNA-binding protein Staufen 2, are required for the development and maintenance of dendritic spine morphology that may be disturbed in cocaine withdrawal. The molecular mechanisms for these neuroadaptations are thought to include alterations in protein turnover at the postsynaptic density (PSD) in dendritic spines that make excitatory synaptic contacts. Third, relapse to cocaine seeking elicited by re-exposure to the selfadministration chamber increases activity-related genes, including BDNF, in the cerebral cortex and/or striatum of abstinent rats. Thus, our hypothesis is that infusion of BDNF into the dmPFC will prevent cocaine abstinence-induced neuroadaptations in dendritic mRNAs, spine morphology, and relapse-induced alterations in activity genes in the PFC and striatum. This hypothesis will be tested by determining (1) the site and mechanism of action of intra-PFC BDNF's suppressive effects on cocaine seeking, and whether infusion of BDNF into the dmPFC (2) alters cocaine-induced changes in gene and phosphoprotein expression during early abstinence, (3) prevents alterations in the expression of dendritic mRNAs, PSD proteins, and dysmorphic dendritic spines in cocaine-abstinent rats, and (3) prevents alterations in cortical activity markers after contextual relapse. Another important goal is to integrate Project 2 with the other projects of the NARC by using overlapping animal models, providing unique information on the same areas of relapse circuitry, and utilizing the expertise of the other Project leaders to advance our understanding of the neurobiology of addiction as a whole.